Since a lithium-ion secondary battery, one type of nonaqueous electrolyte secondary batteries, is light in weight and has a large charging and discharging capacity, it has been used mainly as a battery for portable electronic devices. Moreover, practical use of lithium-ion secondary batteries as batteries for motor vehicles such as electric automobiles is expected. Generally, materials comprising a rare metal such as cobalt or nickel are used as a positive-electrode active material of a lithium-ion secondary battery. However, due to the fact that rare metals are small in the distributed amount, not always easily available and additionally expensive, a positive-electrode active material using a material that replaces a rare metal has been desired. Further, in the case of a positive-electrode active material comprising an oxidized compound, oxygen in the positive-electrode active material is released due to overcharging, or the like, and as a result, an organic electrolyte and a current collector are oxidized and burnt, which may cause firing, explosion, and the like.
A technique of using elemental sulfur as a positive-electrode active material is known. Namely, sulfur is easily available compared to rare metals and is inexpensive, and has a further advantage that a charging and discharging capacity of a lithium-ion secondary battery can be made larger than the present state. For example, it is known that a lithium-ion secondary battery using sulfur as a positive-electrode active material can achieve about 6 times larger charging and discharging capacity than a lithium-ion secondary battery using lithium cobalt oxide which is a general positive-electrode material. Further, sulfur is low in reactivity compared to oxygen, and there is a less risk of causing firing, explosion, and the like due to overcharging.
However, the lithium-ion secondary battery using elemental sulfur as the positive-electrode active material has a problem that a battery capacity is deteriorated through repeated charging and discharging. That is, elemental sulfur is likely to generate a compound with lithium when discharging and since the generated compound is soluble into a nonaqueous electrolyte (for example, ethylene carbonate and dimethyl carbonate and the like) of the lithium-ion secondary battery, the charging and discharging capacity is gradually reduced through repeated charging and discharging due to the sulfur eluting into the electrolyte.
Therefore, in order to prevent a sulfur compound from eluting into an electrolyte, a technique of using a given polycarbon sulfide comprising carbon and sulfur as main component elements has been proposed (JP 2002-154815 A). This polycarbon sulfide is prepared by adding sulfur to a linear chain unsaturated polymer. It is understood that this sulfur-based positive-electrode active material can inhibit the charging and discharging capacity of a lithium-ion secondary battery from being reduced through repeated charging and discharging.
Further, it is also understood that a sulfur-based positive-electrode active material obtained by heat-treating a diene rubber and sulfur is useful for enhancing a charging and discharging capacity (WO 2015-050086).